Bulk composition and phase behaviour of petroleum sourced by the Bakken Formation of the Williston Basin

The Bakken Formation is currently regarded primarily as a self-contained, unconventional petroleum system. While previously viewed as a source for oil occurring in overlying formations, it is now predicted that resources of more than 3.5 billion barrels of oil are trapped intraformationally. New insights into the formation's open v. closed nature are presented here using the physical properties of natural petroleum, source rock characteristics and the numerical modelling of phase behaviour. In the mature western part of the basin petroleum accumulations have been postulated to be continuous in nature, characterized by very short migration distances of indigenous hydrocarbons. This necessitates that the composition and therefore physical properties of the generated hydrocarbons must be controlled by the maturity of the source rock in the immediate vicinity. This assumption is not supported by the clustering of higher gas oil ratios and lighter oil gravities along the locations of the anticlines in the basin. We have used open and closed system pyrolysis techniques to predict the bulk composition of the petroleum generated at different transformation stages, both cumulatively and instantaneously. Based on these predictions the Bakken would contain dominantly undersaturated fluids throughout the basin. Differences in predicted GORs of cumulative and instantaneous models support the conclusion that the reported hydrocarbon compositions cannot completely be explained by a tight self-contained petroleum system. The observed variability of in-place hydrocarbon compositions is readily explained by lateral migration of petroleum in the main middle Bakken carrier, and vertical leakage of emplaced hydrocarbons from the fractured reservoir at anticline locations. This has resulted in the loss of the early generated petroleum, and led to a present-day dominance of late generation products. These results reveal that the Bakker Formation is a partly open petroleum system, at least along the major anticlines of the Williston Basin.